A liquid crystal display device displays an image by controlling On/Off of each pixel in a liquid crystal panel on the basis of a video signal inputted from a personal computer (which will be hereinafter referred to as a PC) or the like and by applying to each pixel a data voltage according to the video signal with a built-in gate driver and a built-in source driver so as to control light transmittance determined by an electrooptical characteristic of a liquid crystal material.
The electrooptical characteristic of a liquid crystal material is determined by the distance between facing substrates, i.e. a liquid crystal gap, as well as the characteristics of the liquid crystal material itself (e.g. birefringent index). More specifically, in a case of a TN (Twisted Nematic) liquid crystal, for example, the transmission intensity I is determined by a per se known expression (1) using a parameter of a product of the birefringent index Δn and the liquid crystal gap d of the TN liquid crystal, i.e. the retardation Δn·d. It should be noted that the liquid crystal gap d is generally designed to be the first local minimum point ((2·Δn·d)/λ=√{square root over ( )}3, λ: wavelength) of the expression (1).
                    I        =                              I            0                    ⁢                                                    sin                2                            (                                                π                  2                                ⁢                                                      1                    +                                                                  (                                                                                                            2                              ·                              Δ                                                        ⁢                                                                                                                  ⁢                                                          n                              ·                              d                                                                                λ                                                )                                            2                                                                                  )                                      1              +                                                (                                                                                    2                        ·                        Δ                                            ⁢                                                                                          ⁢                                              n                        ·                        d                                                              λ                                    )                                2                                                                        expression        ⁢                                  ⁢                  (          1          )                    
FIG. 13 is a graph showing an example of the electrooptical characteristic of a liquid crystal material, wherein the abscissa axis denotes the voltage applied to the liquid crystal material while the ordinate axis denotes the light transmittance of the liquid crystal material. Referring to FIG. 13, a continuous line A denotes a characteristic obtained when the liquid crystal gap d satisfies d=λ·√{square root over ( )}3/(2·Δn), a continuous line B denotes a characteristic obtained when the liquid crystal gap d satisfies d<λ·√{square root over ( )}3/(2·Δn) and a continuous line C denotes a characteristic obtained when the liquid crystal gap d satisfies d>λ·√{square root over ( )}3/(2·Δn), and it is understood that the light transmittance, which is an important quality, changes as the liquid crystal gap d changes, as shown by the above expression (1). Thus, the liquid crystal gap d, which is a parameter for determining the light transmittance of a liquid crystal display device, might be narrower or wider than a designed value due to a variation in manufacture, causing a problem that a desired light transmittance cannot be obtained and an intended gray-level display cannot be provided.
Known as a solution for this problem is a liquid crystal display device comprising a memory for storing a look-up table (which will be hereinafter referred to as an LUT) in which a gray level represented by the inputted video signal is associated with an input level to a liquid crystal panel corresponding to the gray level, whereby a desired gray-level characteristic can be realized by converting a gray level into an input level on the basis of the LUT and correcting specific characteristics of the device (see, for example, Patent Document 1).
The liquid crystal display device disclosed in the Patent Document 1 measures a luminance in each input level with an external photosensor (luminance meter) provided at the front of the liquid crystal panel and evaluates an actual panel gray-level characteristic of the liquid crystal panel. A gray level and an input level, which are to be written into the LUT, are calculated from the measured panel gray-level characteristic and the desired ideal gray-level characteristic, and stored in the LUT.
FIG. 14 is a concept view showing an example of the content of the LUT. The LUT stores a gray level as an index associated with an input level as a value. It should be noted that shown is an example wherein the number of gray levels is 8 bits (256) and the input level is 10 bits (1024), i.e. 2 bits larger than the number of gray levels. The LUT stores a gray level “0” associated with an input level “0”, a gray level “1” associated with an input level “5”, a gray level “2” associated with an input level “8”, . . . , and a gray level “255” associated with an input level “1023”. As shown in FIG. 15, the liquid crystal display device converts an inputted gray level (FIG. 15(a)) of each pixel into an input level (FIG. 15(b)) associated with the gray level on the basis of the LUT, and outputs it.
FIG. 16 is an explanatory view showing the concept of luminance control using the LUT. Referring to FIG. 16, the continuous line denotes an actual gray-level characteristic of the liquid crystal panel and the broken line denotes an ideal gray-level characteristic to be set. The liquid crystal display device converts a gray level X represented by the inputted video signal into an input level Y to the liquid crystal panel on the basis of the LUT to obtain a luminance Q, which gives an ideal gray-level characteristic, so as to realize an ideal gray-level characteristic artificially.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-99238